In recent years, in various displays including liquid crystal displays and electronic devices such as mobile phones, it has been accelerated to make lighter, thinner and more compact displays and elements. With this trend, researches on replacing glass substrates conventionally used with plastic films are actively performed. The plastic film is light and excellent in flexibility. Thus, if the thin plastic film having a thickness of about several μm can be applied to, for example, a liquid crystal display element, an organic electroluminescent element (hereinafter referred to as an “organic EL element”), an electronic paper element and a dispersion-type electroluminescent element (hereinafter referred to as a “dispersion-type EL element”), it becomes possible to obtain an extremely light and flexible functional element.
Among the above functional elements, the dispersion-type EL element is a luminescent element by driving with alternate current voltage, and has been conventionally used for backlights of the liquid crystal displays in such as mobile phones and remote controllers.
Originally, the luminescent element makes an operation easy in a dark place during the night. Thus, as a new application in recent years, it has been attempted to incorporate the dispersion-type EL element in a key input part (keypad) of various devices such as mobile information terminals such as mobile phones, remote controllers, personal digital assistances (PDA), laptop PCs, and the like.
By the way, light emitting diodes (LED) have been applied as the conventional luminescent element for the above key input part (keypad). However, the dispersion-type EL element has been actively used in place of LED, since there are some issues in LED such that LED is emitted from a point source and a luminance in the keypad portion is uneven and an appearance thereof is poor; that although a white or blue luminescent color is generally popular, such a color requires a high cost in LED; and that LED requires a higher electric power consumption than the dispersion-type EL element.
As a method for producing such a dispersion-type EL element, it has been widely known the method of forming the EL element by sequentially screen-printing a phosphor layer, a dielectric layer and a rear electrode layer on a plastic film (hereinafter referred to as a “sputtering ITO film”) having a transparent conductive layer (hereinafter referred to as a “sputtering ITO layer) of an indium tin oxide (hereinafter abbreviated as “ITO”), by generally using a physical film forming method such as sputtering or ion plating.
Here, the above sputtering ITO film is one obtained by forming an ITO single layer which is an inorganic component by the above physical film forming method on a transparent plastic film of polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) so that the thickness is about 20 to 50 nm. Thereby, it becomes possible to obtain the transparent conductive layer having a low surface resistivity of about 100 to 300Ω/□ (ohms per square).
However, there is a problem in the above sputtering ITO layer that microscopic cracks (cracks) easily occur therein since it is a thin film of the inorganic component and extremely fragile. Thus, the plastic film which serves as the substrate is required to have sufficient strength and stiffness, and in an actual condition its thickness is at least 50 μm or more and normally 75 μm or more.
The PET film is widely used for a base film of the above sputtering ITO film, but when its thickness is less than 50 μm, the flexibility of the base film is too high, and the cracks easily occur in the sputtering ITO layer during handling to remarkably impair a conductivity of the film. Thus, it has been the actual circumstance that the thin sputtering ITO film, for example, having the thickness of 25 μm, is not practically used in the device requiring the high flexibility.
In order to further improve the handling, it was also attempted, by using a base film having the thickness of less than 50 μm and lined with a support film having the thickness of about 75 to 125 μm, to form the sputtering ITO film on the base film. In this case, however, the conductivity and the flexibility can not be balanced in the sputtering ITO layer when the support film is delaminated and removed, since the flexibility of the sputtering ITO layer itself is poor.
Also, at present a soft base film, such as a urethane film, is not put to practical use even if its film thickness is 75 μm or more, for the cracks easily occur when the sputtering ITO layer is formed.
By the way, as a property required when the dispersion-type EL element is applied to the above keypad, in addition to the uniform luminance and low electric power consumption as described in Patent Document 1, it becomes important to be excellent in keying durability of the keypad and further click feeling when the keypad is operated.
In particular, in order not to impair the click feeling by incorporating the dispersion-type EL element in the keypad, it is necessary to sufficiently enhance the flexibility of the dispersion-type EL element itself. More specifically, it is necessary to make the thickness of the EL element as thin as possible or use the base film of a flexible material.
However, as described above when the dispersion-type EL element is made using the conventional sputtering ITO film, for preventing the crack in the sputtering ITO layer, it is necessary to make the thickness of the base film at least 50 μm or more to enhance the stiffness. Also the base film of the flexible material can not be used. Thus, when the element is applied to the above keypad, the keying durability is not sufficient yet and the click feeling is not sufficiently good.
Thus, in place of the ITO layer formation by the above sputtering, as the method for forming the relatively flexible transparent conductive layer on the plastic base film as described in Patent Documents 2 to 6, the method in which a coating liquid for forming the transparent conductive layer, composed mainly of conductive oxide microparticles and a binder matrix is applied on the base film and dried, subsequently a compression (rolling) treatment using metal rolls is given, and then a binder component is cured has been known.
This method has an advantage that a filling density of the conductive microparticles in the transparent conductive layer can be increased by the rolling treatment using the metal rolls to widely enhance an electric (conductive) property and an optical property of the film.
However, in the method for forming the transparent conductive layer by these conventional coating methods, in order to obtain the property which is excellent in transparency and conductivity, there is no choice but to set a rolling pressure high. However, in this case, when the thickness of the base film is made thin (e.g., 25 μm or less, particularly 9 μm or less), defects such as a distortion of the base film and an occurrence of the creases tend to occur and as a result, there is such problems that the productivity is reduced and yields of products worsen. Therefore, there has been a limitation in industrial mass production such that in order to prevent them, the base film having the thickness of more than 25 μm (e.g., about 50 μm) is used.
Thus, even in the above method for forming the transparent conductive layer by the conventional coating method, since the base film used is still thick, it has not been possible to sufficiently respond to the flexibility and thinning of the EL element required for the key input parts (keypads).
Also in the flexible functional elements, e.g., the above-mentioned liquid crystal display elements, organic EL elements, electronic paper elements as is the case with the dispersion-type EL element for the above keypad, the film with the transparent conductive layer which is required for producing those flexible functional elements, is formed on the extremely thin base film (plastic film) and is excellent in conductivity, transparency and flexibility has not been obtained.    Patent Document 1: JP 2001-273831-A    Patent Document 2: JP 04-237909-A    Patent Document 3: JP 05-036314-A    Patent Document 4: JP 2001-321717-A    Patent Document 5: JP 2002-36411-A    Patent Document 6: JP 2002-42558-A